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Water on the Move

The hydrological cycle moves water from the atmosphere to the Earth and back again in a process of perpetual motion powered by energy from the sun and by gravity. The cycle’s processes provide the water that flows through rivers, lakes, reservoirs, and aquifers from which many people get their drinking water. Indeed, the water cycle makes life on Earth possible.

Water exists in many different states including snow, ice, liquid water, and vapor—all of which are part of the water cycle.

The endless process is fuelled by the Sun’s heat. Solar energy heats surfaces (the oceans, lakes, reservoirs, and the land surface itself) and creates water vapor that returns to the atmosphere by evaporation. About 86 percent of all global evaporation takes place in the world’s oceans, which promotes a cooling effect on Earth’s climate.

The Sun also melts ice and snow; this water either runs off into rivers, infiltrates into groundwater, or sublimates directly to the atmosphere to become water vapor.

Plants take part in the water cycle by removing water from the soil and discharging it through their leaves as vapor in a process known as evapotranspiration.

Water vapor produced at the planet’s surface rides air currents into the atmosphere, where it encounters cooler temperatures and condenses into clouds. This process is called condensation.

Clouds produce precipitation—the rain and snow that eventually returns most atmospheric water to the Earth so that the ancient cycle can begin anew.

Many water molecules move quickly through the water cycle by either falling into the oceans, which catch 78 percent of all precipitation, or landing on continents where they become part of a runoff system and quickly return to large, surface bodies of water. A percentage of the water molecules that make landfall also infiltrate the soil and become stored in underground reservoirs known as aquifers.

Some water is stored for long time periods by either becoming frozen in ice caps or glaciers or by joining deep groundwater aquifers.

The water cycle supports life on Earth and is intimately linked to our climate. When the climate changes, it alters the water cycle and changes the availability of water resources through space and time.

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